driver: tun: Use new macro SOCK_IOC_TYPE instead of literal number 0x89

The current codes use _IOC_TYPE(cmd) == 0x89 to check if the cmd is one
socket ioctl command like SIOCGIFHWADDR. But the literal number 0x89 may
confuse readers. So create one macro SOCK_IOC_TYPE to enhance the readability.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>